The present invention relates to various aspects of ultrasonic detectors of the type that are used to determine the location of a level within a tank or the magnitude of flow within a conduit.
Ultrasonic detectors may be used to determine the level of the liquid in a tank or the magnitude of the flow of liquid within a conduit. Such detectors have transducers that emit bursts of ultrasonic energy and respond to echoes of the bursts that are reflected by various surfaces. For example, a detector that determines the level of a liquid in a tank transmits energy bursts towards the liquid surface and detects when echoes of the energy bursts are received. The detector then determines the liquid level based upon the time difference between the transmission of an energy burst and the receipt of the corresponding echo. This travel time of an energy burst is directly related to the distance from the transducer to the liquid surface.
The manner in which echoes are detected by ultrasonic detectors varies. In some detectors, the time at which an echo is received is determined by comparing the magnitude of an electrical signal generated from the echo by the transducer to a predetermined value. This comparison may be accomplished by a comparator having a predetermined voltage threshold. When the magnitude of the echo signal surpasses the comparator threshold, the comparator generates an echo-received signal that signifies receipt of the echo and is used to determine the liquid level or flow. Although this manner of detecting when echoes are received is generally satisfactory, it is incapable of precisely determining the exact time of receipt of an echo.
Another method of detecting the receipt of echoes is to use an A/D converter to periodically sample the echo signal. One example of this method is described in U.S. Pat. No. 4,000,650 to Snyder. A significant disadvantage of the Snyder method of detecting the echo signal is that the rate at which the echo signal is sampled by the A/D converter is relatively low. The Snyder detector does not sample in order to detect the peak of the echo signal, it samples in order to locate the echo. For example, in one mode, the Snyder detector is responsive to echoes within a range between 14 to 158 feet, for which the maximum round-trip travel time of an energy burst is stated to be 288 milliseconds. During this time, the Snyder detector samples the electrical signal generated by the transducer only 288 times, or once each millisecond. This rate of sampling, while it may detect the general position of the echo, is simply not sufficient to generate a precise determination of the peak of the echo signal, and thus the precise time at which the echo arrived at the transducer.
Conventional ultrasonic detectors of the type that detect liquid level or flow are generally not capable of locating the presence of multiple targets at various unknown points within a defined range. Such targets might include a single "true" target such as the surface of a liquid within a tank the position of which is being measured and a number of "bogus" targets such as targets corresponding to internal tank structure the position of which is not of interest. Determining the presence of multiple targets within a defined range would be useful for a number of purposes, for example, to aid in distinguishing the true target of interest from bogus targets.
Conventional detectors are also deficient in the manner in which true targets are distinguished from bogus targets. Such detectors generally require the manual measurement of bogus targets and the manual entry of bogus target information. The manual measurement of the distances from the detector at which bogus targets are located may result in errors due to the inaccuracy or limited accuracy of the measurement. The manual entry of bogus target information by an operator may be subject to error. These errors might cause the detector to mistake the true target as a bogus target, or to consider the bogus target to be the true target. In either case, the operation of the detector would be adversely affected.
Conventional detectors are also deficient in that they are incapable of distinguishing between a true echo and a spurious echo associated with the true echo. In certain tanks and/or under certain conditions, such as whether or not the detector is mounted close to a wall of the tank, the true echo generated by the perpendicular path to the liquid surface may be followed relatively quickly by a spurious echo generated by the intersection of the liquid surface and the wall of the tank. This spurious echo may have a larger magnitude since the intersection of the liquid and the wall may be a better reflector of ultrasonic energy. Conventional detectors may not be capable of distinguishing between these two echoes, and may falsely interpret the larger-magnitude spurious echo as the true echo. As a result, the location of the liquid surface as determined by the detector will not correspond to the actual location of the liquid surface. The magnitude of the error will depend on such factors as the tank geometry and the location of the detector within the tank.
Another disadvantage of conventional detectors is that they often use potentiometers to adjust various electrical parameters, such as the gain at which echoes are received. The use of such potentiometers increases the cost of a detector. Another disadvantage is that the position of such a potentiometer may be adjusted by unauthorized personnel, either out of curiosity or in an attempt to adjust or calibrate the detector. Such unauthorized adjustment of the potentiometer is likely to be done incorrectly and adversely affect the operation of the detector, especially in view of the increased complexity of many ultrasonic detectors.